Process for electrolytically graining aluminum lithographic plates



. s as l i 3,073,765 PROCE Fill! ELECTRQLYTICALLY GRAINTNG ALUMHNUMLHTHGGRAPHIC PLATE Ronald Alfred Qharles Adams, Willowhrook Grove,

. London, England No Drawing. Filed Apr. 18, 1960, Ser. No. 22,708 1Claim. (Cl. 204-141) This invention relates to the preparation oflithographic plates made of aluminum or. alloys of aluminum in which thealuminum is the principal element for the purpose of typing,transferring or drawing upon or coating with light-sensitive materials,the said preparation being known in the art as graining.

The invention is a continuation-in-part of my copending application No.760,302, filed September 11, 1958, and now abandoned.

Methods of graining by electrolytic processes have many advantages overother methods of graining but they have only been successful inconnection with the graining of small machine plates of thin metal suchas are used on Office lithographic printing machines, (e.g. plates up to24" in Width and 0.010 thick). Owing to the nature of commercial rolledaluminum, when large aluminum sheets are produced (i.e. plates of sizesgreater than 24" and thickness greater than .010") rolling lines andother surface defects caused during manufacture of the sheets of metalbecome more difficult to avoid and electrolytic graining gives anunsatisfactory surface for lithographic purposes. For example therolling lines tend to grain at different rates leading to irregularwater carrying properties and dark lines on the surface, whilst othersurface imperfections such as occlusions of foreign matter and coarsecrystalline areas give irregular pitting and non-uniform graining whichlead to such lithographic troubles as blind spots" and ink scum. Theextra rolling and smaller areas of the thinner small offset plates helpto minimise these effects, but even in these smaller plates theabove-mentioned defects may be present although not to the extent ofmaking them unusable. Consequently up to the present timeelectrolytically grained lithographic plates have not come intocommercial use except in small machine sizes.

The present invention enables electrolytic methods of graining to beused on any size or thickness of plates with improved results even onthe small offset plates. Moreover the electrolytic process, thus madeavailable, can be very closely controlled to suit the lithographicprocess.

It has now been found that if a very thin but complete layer of metaltogether with any accompanying contamination is removed from the plateby treatment in chemical baths prior to electrolytic graining, the platesurface is left in a uniformly highly active state so that subsequentelectrolytic graining can be carried out more rapidly and uniformly thanhitherto.

According to the present invention the plates are treated in an alkalineetching bath in which the temperature and concentration of the alkaliare chosen so that complete removal of the surface occurs in a period offive seconds to five minutes, and thereafter the plates are treated inan acid etching solution to remove any solid impurities deposited on themetal in the first bath; the plates being finally washed and grained byelectrolytic means using a low voltage alternating current as describedin German Patent No. 700,726, preferably using hydrochloric acid aselectrolyte.

The alkaline treating bath may be a solution in water of an alkali metalhydroxide such as caustic soda. Caustic soda alone may be too violent inaction to obtain a uniform action and the bath preferably includes abuffering agent such as trisodium phosphate to modify the action of thecaustic soda. The concentration is not critical and will be chosen withregard to temperature of chemically clean and uniformly reactive metalsurface,

but owing to impurities normally present in commercial aluminum, a fineloose deposit may be left on the 'sur-,

face consisting'largely of iron and silicon which is then removed in thesubsequent acid bath. A suitable acid bath consists of a mixture ofnitric and hydrofluoric acids.

(the concentration is not critical) which will give the desired resultswithout pitting. The nitric acid concentration may be 1% v./v. to 50%v./v. of the concentrated acid and the hydrofluoric acid may be 1% v./v.to 20% v./v. of the concentrated acid. Nitric acid alone may be used ifthe aluminum sheet is free from silicon. The etching time should be fiveseconds to five minutes according to the proportions of acid and thetemperature should be at room temperature.

After the acid bath and after rinsing thoroughly the sheet iselectrolytically grained in known manner, e.g. using dilute hydrochloricacid as follows:

The alternating potential between pairs of plates should be 550 volts,preferably 7-15 volts, with the plates spaced apart from 1 inch to 12inches, preferably 2-6 inches and with an electrolyte consistingessentially of dilute hydrochloric acid (0.75 to 2.0 Normal) thoughother soluble chlorides such as sodium or magnesium chlorides may bepresent. The temperature should be about'room temperature (e.g. 4090 F.)The sheet is then rinsed thoroughly after which it is preferable topassivate the grained surface by immersion in a passivating solutionsuch as dilute ammonia, dilute sodium silicate or dilute ammonium orother dichromate prior to the final rinsing and drying.

As a result of this sequence of operations a grain is produced which iscompletely uniform and which is of outstanding behaviour forconventional lithographic platemaking and machining. It is greatlysuperior to the conventional marble graining or sand-blasting and asexplained earlier enables qualities and sizes of metal to beelectrolytically grained which would give unusable grains if theelectrolytic graining treatment were applied directly to the untreatedplates. Moreover the process according to the invention produces a moresatisfactory grain in cases such as on thin metal which may give arelatively satisfactory grain even when the electrolytic graining isdirectly applied.

It has also been found that the grain produced according to theinvention is specially suitable for coating with light-sensitive organicresins, particularly those which are soluble in organic solvents such asthe polymeric cinnamic esters, to give a pro-sensitized lithographicplate of outstanding stability and durability. Furthermore, the grain issuitable for subsequent anodising and sealing as described in ourBritish Patent No. 781,814 and which may also be pro-sensitized ifdesired.

I claim:

A process for graining lithographic plates made of a metal selected fromthe group consisting of aluminum and aluminum base alloys, which processcomprises the 0 following steps in the order named:

(a) immersing the plates in an aqueous solution of Patented Jan. 15,less 3 caustic soda and trisodium phosphate containing 0.25% w./v.sodium hydroxide and 1% w./v. trisodium phosphate at a temperature above60 C. for a period of. 5 seconds to 5 minutes;

(b) immersing the plates in an aqueous solution of nitric acid andhydrofluoric acid containing 1% v./v. to 50% v./v. of concentratednitric acid and 1% v./v. to 20% v./v. of concentrated hydrofluoric acidat room temperature for a period of 5 seconds to 5 minutes;

(c) rinsing the plates;

(d) electrolytically graining the washed plates in a dilute aqueoussolution of hydrochloric acid of a concentration 0.75 to 2.0 Normal withan alternating 4 current of 5-50 volts between pairs of plates at atemperature of 4.5-32 C.; (e) rinsing and passivating the grainedsurface with dilute ammonia prior to final rinsing and drying.

References Cited in the file of this patent V UNITED STATES PATENTS1,705,944

Siegmund Mar. 19, 1929 2,171,546 Kappes Sept. 5, 1939 2,344,510 HagelinMar. 21, 1944 2,347,572 Martin Apr. 25, 1944 2,507,314 Mason May 9, 19502,541,901 Zademach Feb. 13, 1951 2,811,426 Mason Oct. 29, 1957

